1. Field of the Invention
This invention relates to a fixing apparatus provide in an image forming apparatus such as an electrophotographic printer, copying machine and the like, and more specifically, a fixing apparatus equipped with a mechanism for coating a rotating member for the pressure-thermal fixing process with a releasing agent to prevent the offset phenomenon.
2. Description of the Related Art
An image forming apparatus such as an electrophotographic printer, copying machine and the like is equipped with a fixing apparatus which fixes undeveloped toner images on sheets of recording media. One of the most widely used fixing apparatuses today is the pressure-thermal type.
A fixing apparatus based on the pressure-thermal fixing process comprises a fixing roller and a pressure roller, which are opposed to each other, and a tubular heater, such as a halogen lamp, which is placed in the center shaft of the fixing roller. The heat generated by the halogen lamp is radiated uniformly on the inner wall of the fixing roller, so that the temperature distribution on the outer wall of the fixing roller becomes uniform in the peripheral direction. The outer wall of the fixing roller is heated up to a temperature appropriate for fixing (e.g., 150-200.degree. C.). Under such a condition, the fixing roller and the pressure roller rotate in directions opposite to each other while being in contact under pressure and nipping a sheet that holds the toner. In the nipping area where the fixing roller and the pressure roller are in contact under pressure, the unfixed toner on the sheet melt due to the heat transferred from the fixing roller and is fixed on the sheet with the help of thepressure exerted by the two rollers.
The fixing apparatus based on such a process tends to cause a transfer of a portion of the unfixed toner image on the sheet to the fixing roller side since the toner holding surface of the sheet makes a direct contact with the fixing roller surface. As a result, the molten toner adhered to the fixing roller may smear said sheet by being transferred back to the rear end of the sheet, or smear the next sheet by being transferred to it. This problem, known as the offset phenomenon, is a common problem of this type of apparatus.
Therefore, the fixing apparatus based on the pressure-thermal fixing process, in particular, the color fixing apparatus, has a mechanism for coating the fixing roller with a releasing agent to help the separation of the toner from the fixing roller. As the releasing agent, silicon releasing agent (hereinafter referred to as "oil") is normally used.
In such an oil coating mechanism, the oil on the fixing roller surface has to be replenished as the oil on the fixing roller is taken away by the sheet when the sheet passes through the mechanism for replenishing the oil to said oil coating mechanism.
Such an oil replenishing mechanism can be a mechanism which coats more than enough amount of oil on the fixing roller and collect the excessive amount by means of an oil recovery mechanism so that the collected oil can be recycled (refer to JP-A-05-265346), or a mechanism that which allows a small amount of oil to be seeped out from a roller which holds a predetermined amount of oil and the entire unit is replaced after a certain amount of oil has been consumed (refer to JP-A-02-23382).
The former case, however, has a disadvantage that too much oil is taken away by the sheet. Also, since it requires an oil recovery mechanism, it is disadvantageous from the cost standpoint. On the other hand, the latter case has a limitation to the amount of oil retained If it is attempted to hold a large amount of oil to extent the life, the apparatus becomes too large. In other words, in order to hold a large amount of oil, the roller diameter has to be enlarged, for example, which results in increasing the size of the apparatus itself. Also, as shown in FIG. 1, a large amount of oil seeps out when the oil containing roller is first set, the seepage gradually decreases as the number of sheets processed increases; in other words, the amount of seepage changes with the amount of oil remaining on the roller. Therefore, the oil seepage is unstable and, in other cases, it may even cause a problem of image noise such as oil streaks and offsets due to oil leakage from the fixing roller.
One of the oil coating mechanisms of the prior art comprises an oil tank which extends along an axial direction of the fixing roller and a felt pad or roller which is arranged to be immersed in the oil contained in the oil tank, so that the oil can be supplied to the oil coating roller which maintains contact with the fixing roller by means of the felt, etc. (refer to JP-A-54-76234). However, such an oil coating mechanism may cause oil spillage or oil smearing because it has a large amount of oil in an open oil tank. Moreover, if the image forming device itself vibrates or is installed in a tilted position, the fixing apparatus, hence the oil coating apparatus as well, vibrate or tilt, which tends to cause oil spillage. Moreover, in an oil coating mechanism which supplies the oil in a planar fashion to the oil coating roller, it is difficult to provide a uniform amount of oil on the surface of the fixing roller relative the axial direction of the fixing roller, and this may be a cause of image noise problems such as oil streaks.
Therefore, another type of oil coating mechanism has been proposed in recent years where multiple oil discharge ports are provided on a supply nozzle so that the oil is supplied in multiple droplets on the oil coating roller (refer to JP-A-06-274061 and JP-A-07-210025). In such a processing format where the oil is supplied form a nozzle, a sealed oil tank can be used, so that any oil spillage from the tank can be prevented. Also, by adjusting the operating time period of the oil pump, the oil supply amount can be arbitrarily selected from small to large thus giving an advantage of broadening the range of oil supply amount.
On the other hand, as shown in FIG. 2A, it is difficult to control the growth of droplet 4 accurately. This is because oil 3 adheres to and spreads around the vicinity of discharge port 2 and joins with the oil 3 that is leaving adjacent discharge ports 2 causing droplet 4 to grow. As a result, even though the total quantity of oil 3 discharge is controllable, it is difficult to controllable, it is difficult to control accurately the size and growth rate of droplet 4 generated at each discharge port 2.
Moreover, if the supply nozzle 1 is tilted, as shown in FIG. 2B, oil 3 that has left discharge port 2 flows along the surface of supply nozzle 1 in a descending direction and joins with oil 3 that has left other discharge ports 2. This oil 3 therefore drips at a point offset from the intended dripping point and the amount of droplet 4 becomes large than the intended amount.
Consequently, in a constitution described above, which depends on the growth of droplets from the discharge ports of the supply nozzle, the oil from the discharge ports of the supply nozzle, the oil from the discharge ports tends to flow down along the surface of the nozzle if the fixing apparatus is tilted, causing droplets to gather at lower points of the tilted nozzle so that they fail to grow in the intended point. As a consequence, the oil drops at unintended places or the droplets fail to grow to an intended size, causing fluctuation of the discharge amount and image noise problems such as oil streaks and offsets.
Also, in case of a supply nozzle having multiple oil discharge ports on a pipe extending along the axial direction of the oil coating roller, a technique of changing diameters of discharge ports along the longitudinal direction in order to obtain a uniform oil discharge quantity along the longitudinal direction has also been proposed.
However, since the diameters of discharge ports have to be changed substantially along the longitudinal direction, the maximum discharge port diameter becomes substantially large as the nozzle length becomes longer. As a result, the diameter of the nozzle itself has to be larger, making the oil coating mechanism larger and consequently making the fixing apparatus larger. Another problem is that the process of forming discharge ports becomes complex as the diameters of the discharge ports vary, thus causing an increase of manufacturing cost. Moreover, if the fixing apparatus is under vibration or tilted, the varying diameters of the discharge ports aggravate the oil leakage and vary the amount of the oil discharge amount, discharged thus creating image noise problems such as oil streaks and offsets.
Another type of oil coating mechanism of the prior art comprises an oil coating roller which rotates and maintains contact with the fixing roller surface under pressure, and an oil supply roller which holds a releasing agent in its cylindrical body and maintains contact with the oil coating roller surface under pressure (refer to JP-Y-03-10525).
In such an oil coating mechanism, the oil which is applied to the fixing roller surface is supplied from the oil supply roller by way of the oil coating roller. This makes it possible to delegate the function of contacting under pressure and uniformly coating the fixing roller surface with the oil and the function of holding the oil and controlling the coating amount to the oil coating roller and the oil supply roller, respectively. Since the oil coating roller exists between the oil supply roller and the fixing roller, the oil coating roller prevents the fixing roller's heat from being directly transferred to the oil supply roller, thus reducing the chances of evaporating the releasing agent contained therein.
In the oil coating mechanism of the fixing apparatus described above, the oil supply roller comprises a cylindrical metal core with small holes formed on its outer surface and a porous sheet which covers it, and the oil is sealed in the inside of the metal core and, as such, the oil is always ready to permeate into the porous sheet through small holes of the metal core. In other words, the porous sheet of the oil supply roller is always impregnated with the oil and as the oil is consumed, it is replenished from the inside of the metal core through the holes. Therefore, in order to prolong its useful life, the size and number of the small holes on the metal core have to be limited.
However, in color copying machines manufactured in recent years, the oil used for making each A4-size copy is more than it used to be; for example, it requires more than 10 mg in some cases. On an oil supply roller such as the one described above, there has always been a problem in providing a sufficient response, i.e., to supply a sufficient amount of oil quickly. If the amount of oil applied on the fixing roller is insufficient, it leaves some parts of the roller uncoated, resulting in offset phenomena or contamination of the oil supply roller as a result of odd objects adhering to it.
On the other hand, increasing the diameters and the number of small holes formed on the metal core in order to increase the amount of oil to be coated on the fixing roller creates a condition wherein the porous sheet is impregnated with excessive amount of oil constantly and thus creating oil streaks on the sheet. Also, in such a case, the oil supply roller has a relatively short usage life, hence increasing the frequency of part replacement.
Yet another type of oil coating mechanism of prior art is the one comprising a coating roller which coats the fixing roller with oil, an impregnating material such as felt to supply the oil to the coating roller, a tank unit which stores the oil, a transferring passage which transfers the oil stored in the tank unit, and a pump unit provided on the transferring passage in order to transfer the oil.
As one of such an oil coating mechanism, a mechanism has been disclosed (refer to U.S. Pat. No. 4,193,681 and JP-B-63-11669), wherein an oil holding member made of felt makes contact with the coating roller at one end and immerses itself in the oil contained in an auxiliary tank on the other end, while the main tank supplies an excessive amount of oil to the auxiliary tank, thus allowing the excessive oil overflowing from the auxiliary tank to be collected in the main tank.
There is yet another type of mechanism of prior art, wherein the pump unit has a pair of tubes equipped with a ball valve, which are connected with a flexible tube made of urethane rubber and the like having an excellent restoring capability, and an pressuring number, such as a cam, which presses the rubber tube (refer to JP-A-54-76234). This pump unit causes an elastic deformation to the rubber tube be means of the pressuring member to push out the oil contained in the rubber tube and, by removing the pressure, allows the rubber tube to restore its original shape to such the oil from the tank.
On a fixing apparatus of an image forming apparatus which is capable of printing or copying full color images, it is normally necessary to coat the fixing roller with a larger amount of oil compared to the monochromatic image fixing apparatus. On the other hand, it is necessary to reduce the amount of oil consumption in order to prolong the maintenance interval or to reduce the waste of material. In order to accomplish this, it is desirable to replenish the required amount at the required moment, so that it is necessary to establish a technique of supplying a small metered amount of oil intermittently and steadily.
However, the oil coating mechanism of the prior art which supplies the oil excessively and collects the excessive amount of oil is not constituted to supply the oil intermittently, although it can supply the oil continuously. Therefore, it is necessary to provide a means of intermittently supplying the oil from the auxiliary tank to the coating roller in addition to the means of supplying an excessive amount of oil from the tank to the auxiliary tank, thus causing a complexity to the constitution.
Moreover, the above-mentioned pump unit of the prior art has a very simple constitution using a ball valve, is inexpensive and shows a stable oil supplying capability in case of a continuous supply. However, since the ball valve, is inexpensive and shows a stable oil supplying capability in case of continuous supply. However, since the ball valve is not enough to stop the back-flow of oil, the oil level in the transfer passage drops substantially which the pump is at a stand-still. Consequently, if the oil has to be transferred intermittently, the oil level in the transfer passage may change each time when the pump is restarted, so that the amount of oil transferred may vary even if the pump is operated the same number of times and for the same number of time periods. This created a problem that the amount of oil coated on the fixing roller can not be stabilized.
Furthermore, in the aforementioned oil coating mechanism (refer to JP-A-06-274061 and JP-A-07-210025) which supplies the oil in droplets from the multiple oil delivering holes provided on the supply nozzle, an amount oil which exited a delivering hole and remained in the vicinity of the delivering hole flows down along the surface of the supply nozzle; if the supply nozzle is tilted, then the oil combines with other amounts of oil remaining in the vicinities of other delivering holes and eventually reaches the lower end of the supply nozzle. The oil that gathered at the supply nozzle end may leak to the outside of the fixing apparatus and soil the hands and/or clothes of the operator, or leak to the other parts of the mechanism causing contamination to those parts.
It is also common for a typical oil coating mechanism of the prior art to have a blade made of metal or rubber which abuts the oil coating roller in order to accomplish a uniform coating of the fixing roller and to stabilize the amount of coating.
This blade, which is intended to produce a uniform coating, often collects odd particles on the edge of the blade edge. When odd particles are collected on the blade, the amount oil coated in the area where odd particles exist is less than areas surrounding it, thus creating uneven coating on the fixing roller. This tends to cause offset phenomena and oil contamination.
Another process that has been disclosed (refer to JP-B-59-29874) comprises an oil coating roller for coating a fixing roller with oil and an oil supply roller, which is designed to contact with the oil supply roller, which is designed to contact with the oil coating roller under pressure, whereby odd particles are transferred due to the surface energy difference between the two rollers and discarded thereafter.
However, the process mentioned above is designed to remove the odd particles after they have attached to the roller is not intended to eliminate the cause of adherence of odd particles to the roller. Moreover, the odd particles move only to the oil supply roller, which is located upstream, and cause an uneven oil transportation from the oil supply roller to the oil coating roller. As a result, it still causes an uneven coating on the fixing roller